The present invention relates to an extrusion die for forming thin-walled honeycomb structures from extrudable materials such as glasses, glass-ceramics, ceramics, plastics, metals, cermets and other materials. In the ceramic arts, such dies are used for the extrusion of ceramics dispersed as powders in shapeable (plastic) extrusion batches to provide extruded green bodies of complex honeycomb shape.
Thin-walled ceramic honeycomb structures with multiple parallel through-channels or cells display utility in a variety of applications. For example, such structures exhibit utility as catalytic converters in the exhaust system of internal combustion engines. They also exhibit more general utility as catalyst carriers, filter bodies, and thermal regenerators or heat exchangers.
Dies used for the extrusion of ceramic honeycombs commonly have shallow, criss-crossing and interconnecting slots on the downstream or exiting die face from which the ceramic batch emerges and which during emergence form the webs or sidewalls of the cells of the honeycomb structure being made. To supply the batch material to these slots, feed holes are provided in the opposite or upstream die face which connect with and feed batch material to the slots.
In common production dies the feed holes are aligned with the intersections of the slots on the outlet face of the die. This is because the intersections generally require larger proportions of the batch material for proper slot filling and web formation in the extruded shape. Some dies have a feed hole at every intersection, while other dies have holes at alternate intersections. Alternating hole patterns using fewer holes of larger diameter can be advantageous in that the dies are easier and less costly to produce, and are more resistant to bending deformation under high extrusion pressure.
Dies are also occasionally made with the feed holes aligned with the central portions of the slot segments, eg., midway between the slot intersections. This feedhole positioning can improve the strength of the "pins", which are the projecting islands of metal bounded by the slots on the exit face of the die defining the channels in the honeycomb material extruded from the die.
A variety of die configurations for extruding honeycomb bodies of both triangular and square cell cross-section are known. U.S. Pat. No. 1,874,503, for example, discloses a triangular cell extrusion die wherein the feed holes supply batch material to the intersections of the triangular slots, this die being used for the extrusion of candy. Unexamined Japanese Patent Publication No. 50-29922 describes extrusion dies for the continuous manufacture of ceramic honeycombs having channels or cells of triangular cross-section which comprise feed holes supplying either the slot intersections or the central portions of the slots.
Dies of alternating feed hole design are also known. U.S. Pat. No. 4,741,792, for example, discloses a rectangular cell die configuration for extruding honeycomb ceramic heat exchanger bodies wherein the feed holes are positioned at alternating slot intersections In this design, only two of the four corners of each extruded cell are formed by the direct flow of batch material thereto. The other corners of each cell are formed by lateral flow of the extrudable batch material within the slots to achieve the necessary web knitting at such other corners.
More complex arrangements of holes and slots are shown in U.S. Pat. No. 4,902,216, which describes dies comprising both primary and secondary discharge slots. In dies of this design, the secondary slots, which typically differ in depth from the primary slots, are supplied with extrudable material by the primary slots, rather than by feedholes communicating directly with the secondary slots.
One problem associated with the production of extruded bodies from dies of the types shown in these patents relates to the thermal shock resistance of the bodies in applications involving use at high temperatures. Thus extruded honeycomb products composed of fired ceramics, or even of sintered metals, are subject to severe thermal stress in the course of heating and cooling from high temperatures. This is particularly true where, as is frequently the case, the heating and cooling is not uniform across the body.
Thermal shock problems in these honeycomb bodies are aggravated by the fact conventional dies and honeycombs produced therefrom feature arrays of square, rectangular or triangular cells based on long straight webs traversing the entire cross-section of the bodies. Honeycomb product configurations based on such straight continuous webs can be efficiently made since dies with long straight slots are economical to fabricate. Unfortunately, however, such cell arrays do not exhibit optimum thermal shock resistance, due in part to the limited flexibility of long straight webs which cannot readily relieve stress by bending.
In principle, honeycomb bodies having cell configurations wherein long webs are absent would offer advantages in thermal shock performance. Examples of such cross-sections, which could exhibit greater flexibility than conventionally packed triangular, square, or rectangular cells, include hexagonal, rhombic, and staggered rectangle cell cross-sections. Attempts to make such structures have included specialized die modification techniques such as disclosed in U.S. Pat. No. 4,127,691. In accordance with that patent, selected segments of long discharge slots on the discharge face of a conventional die are plugged to pattern the discharge flow into a honeycomb body with a staggered or "flexible" rectangular cell cross-section. Although extruded products of this type exhibit greater inherent flexibility, due to the absence of webs running continuously across the cell array, dies with plugged or other elaborately patterned discharge slot configurations are both difficult to fabricate and expensive. In addition, difficulty can be encountered in designing a feedhole array sufficiently homogeneous in layout to assure uniform continuous batch extrusion characteristics in the die.
Accordingly, it is a principal object of the present invention to provide a novel and economical extrusion die, and method for using it to make honeycomb bodies, which can produce extruded green ceramic honeycomb preforms having a "flexible" cell configuration. It is a further object of the invention to provide an extrusion die design incorporating an arrangement of straight slots and feedholes such that the cross-sectional cell shape and cell arrangement of extruded bodies produced by the die do not comprise any long straight webs.
It is a further object of the invention to provide an extruded honeycomb body having a cross-sectional cell shape and cell arrangement substantially free of long straight web segments. It is a further object of the invention to provide an extruded honeycomb body having a cross-sectional cell shape and cell arrangement in the configuration of a plurality of nested rhombuses.
It is a further object of the invention to provide an improved method for making an extruded honeycomb body having a cross-sectional cell shape and cell arrangement in the configuration of a plurality of nested rhombuses.
Other objects of the invention will become apparent from the following description thereof.